For me I think its because they were scared. They didn't want to disagree with the man who WILL kill you. Jinwoo voted against because he didn't know the trauma the towns people went through. Not to metion gyuwon having favored jinwoo to not harm him due to a disagreement
The people fear him so they do what he says to stay alive. They fear him. So I guess they killed him to save their skin.
Yeah everyone in town collectively agrees that they need to show Guwon they're on his side for everyone to stay alive, thats why they believe every vote has to me unanimous. I don't know how truly necessary that is but that's what they believe is safest
I still dont get y every town person voted to kill that kid's father